The invention relates to a method and an apparatus for manufacturing tubes, preferably composed of high-tensile steels or special alloys, using the cold pilger rolling method with two rolling stands which can be moved backward and forward, at least in opposite directions at times, in the rolling direction by means of crank drives and have rollers which are calibrated in a tapering manner which, driven via toothed racks via cogs, roll over the material to be rolled, with an alternating rotation direction.
A major cost element in the manufacture and operation of cold pilger rolling mills results from the rotation and advance devices required as well as the feed devices which are essential for the cold pilger rolling process. The performance-to-cost ratio can be improved considerably if a significant performance improvement can be achieved while maintaining these devices and without reducing the stand speed. One way to achieve this is to increase the forming work per stand stroke and rolling stand, which results in a considerable improvement in performance for only a minor increase in investment costs. This statement relates to cold pilger rolling mills in general and, in particular to cold pilger rolling for relatively small tubes composed of high-tensile steels or special alloys.
Known cold pilger rolling mills in which rolling is conventionally carried out in one (strand), suffer from relatively high investment costs for relatively low performance in comparison with modern drawing methods. In order to improve the performance, it has been proposed that cold pilger drawing mills be operated with a plurality of parallel (strands), for example two to four. However, such a method of operation means greater stand weight with reduced speed and an increased complexity for the feeding and the rotation advance devices, while obtaining rolled tubes having tolerances which leave something to be desired.
The use of so-called tandem cold pilger rolling mills have already been attempted. In that system, two roller pairs are combined sequentially in one stand. Once again, the relatively high stand weight and low speed are evident in a poor cost-to-performance ratio. Both sets of rollers roll the advanced tube volume at the same time. The rolled tube length from the first set of rollers is supplied to the second set of rollers during the advance. This can result in bulging problems in the tube, associated with reductions in performance and quality.
Finally, FIGS. 5 and 6 of German Patent Specification 604 909 show a cold pilger rolling mill which has two rolling stands which can be moved backward and forward in opposite directions at times in the rolling direction. The rolling stands are moved by means of crank drives. The rollers are driven with an alternating rotation direction via toothed racks. The known arrangement provides for the billet to be reduced in diameter exclusively in the first rolling stand, and without any mandrel, so that the wall thickness of the tube can then be reduced using a mandrel in the second rolling stand. The arrangement of the crank drives is chosen such that the movement sequences of the two rolling stands, together with the movement of the mandrel rod and the gripping of the rollers, allow the tube to be advanced in a specific manner.
Although the explanation of the method of operation of the known rolling mill arrangement does not make it possible to identify the precise method sequence when tubes are being rolled, it is, however, evident that this rolling mill could, in any event, be operated at a low performance level which was admittedly adequate in the past, but no longer meets the requirements for a modern cold pilger rolling mill. The hollow rolling in the first stand leads to deterioration of the inner surface that is currently unacceptable and results in only a small, if any at all, performance improvement, since the significant wall reduction is carried out exclusively in the second stand.
The object of the present invention is to provide a cold pilger rolling method and an apparatus for manufacturing tubes, in particular composed of high-tensile steels or special alloys, using the cold pilger rolling method, in which a significant improvement in the rolling performance over conventional rolling mills is obtained with as little additional mechanical complexity as possible and without any reduction in quality.